Modifying a Three Brush Dynamo System to Solid State Regulation

Jim Thomas has recently modified the charging system on his TA. He describes the benefits and how he went about the task.

I chose to modify the original three brush dynamo and cut-out charge regulator to a modern solid state system for improved functionality and reliability (no point in using a 2 or 3 bobbin mechanical regulator). Why do this modification?

  • Can use LED bulbs and other electronics safely without trying to find a compromise 3rd brush adjustment for each load

  • No sticking cut-out points, dynamo damage or fire risk

  • Longer battery life and less topping up (no overcharging)

  • Can use any type of lead acid battery

I chose the DVR3 regulator from ‘Dynamo Regulators Ltd’, but before I could purchase this I had to check that the field resistance of the C45 NV 3-brush dynamo on my TA was greater than 2.5 ohms – it was 3.7 ohms, therefore safe to proceed. The DVR3 regulator allows one to choose the current limit of the dynamo. The original is capable of a maximum output of 13 amps but I chose an 11 amps limit as I intend to fit LED bulbs and rarely go out in the rain at night. For a few pounds it is possible to purchase a regulator with a custom current limit.

I wanted to hide the regulator and any new wiring and that proved easy to do. I also kept the car positive earth as I quite like the quaintness of this. The DVR3 fits snugly in the back of the CJR3 cut-out once the field resistors are removed. It is simple to return the system to a relay cut-out too. The instructions that come with the DVR3 are very clear and should be used with the notes below that cover the specific modification for the MG TA CRJ3 cut-out. These wiring modifications are quite simple and can be completed in a few hours with the dynamo and cut-out left mounted on the car:

Modify the dynamo for 2-brush operation:

  • Disconnect the battery

  • Loosen the dynamo mountings, remove the fan belt and open the inspection cover

  • Remove the 3rd brush (small thin one) and spring

  • Add a link from the 3rd brush where the field wire is terminated to the ground connection on the grounded main brush (see photo 1) – both had screw connections on my dynamo.

Photo 1 – Dynamo being modified for 2-brush operation.

Modify the CJR3 cut-out unit:

  • Remove the two machine screws and nuts holding the unit to the bulkhead

  • Remove the field resistors with their terminal screws, asbestos and the perforated screen

  • Disconnect the cut-out. I could de-solder one wire and this disconnected both the ‘shunt’ and ‘series’ coils in the cut-out. Other cut-outs have screwed connections that can be isolated

  • Remove the wires from the F1 and F2 terminals that go to the ignition switch on the dash and insulate the open ends. Leave the yellow ‘D’ wire connected to drive the warning light

  • Move the dynamo field wire from F2 to F1 (this is to allow the original dynamo field fuse to become the new fuse that protects the regulator)

  • Add a link from F2 to A1 (where the supply to the car wiring originates)

  • Replace the old field fuse with one that is a little larger than the rated maximum output of the DVR3 regulator – I used a 13 amp domestic fuse to test the system but have now soldered some 15 amp fuse wire into the original glass fuse

  • Mount the DVR3 regulator on the bulkhead. I used the sticky pad that comes with the unit to locate it as I pushed the CJR3 cut-out back on its original mounts. For a better mounting, I threaded the holes in the DVR3 unit and drilled two small holes in the bulkhead and machine screw mounted the unit firmly with heat paste between the top and the bulkhead

  • Wire up the unit as shown in the diagram – also see photo 2

  • Insulate the bare tracks (use a piece of plastic). Re-fit the CJR3 thus concealing the DVR3

  • Replace the fuse and cut-out lids (even put back the lead seal that was originally wired around the spring clip holding the cut-out lid)

  • Reconnect the battery and switch on. The charge warning light should come on. Start the engine and see what smooth and well-regulated current looks like on the ammeter! The voltages look good too thus enabling the battery to fully charge rapidly for this first time. There is positive charge almost at tick-over, the lights are brighter and the starter sounds a bit more powerful.

A really worthwhile, simple, non-intrusive modification I believe. Jim Thomas

Photo 2 – showing the wiring (see also wiring diagram).

Photo 3 – Job done!

Ed’s note: This article was published in the February issue of the MG OCTAGON CAR CLUB’s ‘Bulletin’.

Dynamo Regulators Ltd 75 Titchfield Park Rd,Fareham PO15 5RN Telephone 01489 880552.

Classic Dynamo and Regulator Conversions Ltd CDRC Ltd Unit 4a, Rear of Bridge Street Garage, Bridge Street, Saxilby, LINCOLN LN1 2PZ Telephone 01522 703422

One thought on “Modifying a Three Brush Dynamo System to Solid State Regulation

  1. Tim Parrott says:

    This modification still leaves much of the original cut out connected. The CJR3 on my TA was replaced by an RF95 however I got a 6 volt CJR3 and fitted the DVR controller. I removed the metal strip connections to the cut out and left the coil inplace as decoration. I soldered the ends of the metal strips to wires and brought them to a terminal block. I then connected the DVR3 wires to the new terminals.
    I used the F1 and F2 wires for other purposes. One as a supply for a USB connection and the other for an oil pressure warning light.
    I believed that I had a TD 2 brush Dynamo but it turns out to be a converted TA one. Plus I failed to check the resistance. It is running OK with a 22 amp controller but I will keep checking it.
    I regret not switching to negative earth as LED bulbs are more available and cheaper so at some time I may swap it over and drop the power rating.

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